A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and burned for generating hot combustion gasses. These gasses flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide thrust for powering an aircraft in flight. Typically, a hollow shaft is axially located within the gas turbine to connect the compressor and turbine for rotation therewith. The shaft may be interconnected to rotary components of the engine by a series of radially spaced spline teeth or “splines.” The shaft is typically manufactured with a forging, machining and hardening process.
A gas turbine engine rotates in a single direction during operation and is a delicately balanced machine. As the engine is operated normally, the splines on the shaft transmit loads and experience wear, generally on a single face. Typically, a spline is a thin member that interfaces with a mating spline on an adjacent part. The face of each spline that transmits the torque from the turbine to the compressor typically wears in a predictable pattern. This wear on the load transmitting, or pressure, face of the splines can cause loss of material stock, the creation of a stress riser on the pressure face and reduction of service life of the part, eventually necessitating an engine rebuild. Excessive spline wear could result in failure of both the splines and the engine.
During an engine rebuild, the shaft is inspected to determine the amount of wear on various contacting surfaces, including the spline and seal teeth. The shaft dimensions are compared to a predetermined set of dimensions and tolerances. A shaft that is not within acceptable tolerances of the predetermined dimensions is typically scrapped and a new shaft is inserted into the rebuilt engine. One reason for scrapping the shaft is the lack of adequate repair methods for the worn spline teeth. Typically, the wear experienced by the splines is only a few thousandths of an inch on the pressure face of the splines, with negligible wear on the opposite faces and tops of the splines.
Efforts to repair the worn face of shaft splines by welding a filler material to the worn face and remachine this face have resulted in a repaired shaft whose dimensions are restored, but have inadequate material properties on the pressure face of the spline. One of these inadequate properties is an undesirable change in grain size within the heat affected zone (HAZ) of the weld area. The HAZ is the region(s) surrounding a weld that are measurably affected by the welding process. If sufficient heat is transferred into a welded substrate, microstructure grain size can increase. A larger grain size is associated with lower fatigue life and results in a material microstructure that is more susceptible to cracking and will withstand less cyclic stress. Fatigue cracking on the pressure face of splines are a known cause of premature failure.
Accordingly, there is a need for a method of repairing the spline teeth of a power transmission shaft of a gas turbine engine that overcomes the inadequacies of the prior art.